1. Field of Endeavor
The present invention relates to an improved sealed electric generator.
2. Brief Description of the Related Art
With reference to FIGS. 1 and 2, sealed electric generators 1 include a casing 2 that houses a stator 3 and a rotor 4 rotatable therein.
The rotor 4 has a shaft 5 that is supported by bearings 6. Seal rings 7 are provided to seal the inner of the casing 2 (usually containing H2) against the outer environment.
Generators 1 are known to also include an exciter 8 that generates DC electric power to be fed to rotor windings 9.
The exciter 8 is normally located outside of the casing 2 and, in order to supply the DC electric power to the rotor windings 9, the shaft 5 is provided with a duct wherein a lead 10 is housed.
The lead 10 is connected at one side to the exciter 8, and at the other side to the rotor windings 9.
In addition, since the inside 11 of the casing 2 is filled with H2, seals must be provided in order to prevent the H2 from reaching the exciter 8, because H2 when mixed with air generates explosive mixtures, that may explode in presence of sparks (caused by the exciter).
In particular, FIG. 1 shows a first type of generator in which the exciter 8 supplies electric power to slip rings 12 provided on a slip ring shaft 5a connected to the shaft 5.
As shown, in this embodiment the duct and the lead 10 extend in the shaft 5 and slip ring shaft 5a. 
The slip rings 12 are connected to the lead 10 via radial rods 13 (inserted in radial holes connected to the duct); the lead 10 is then connected to the rotor windings 9 via radial stalks 14 (also inserted in radial holes connected to the duct).
In these generators the seals are traditionally located in position A between the radial holes and the corresponding radial stalks 14; in addition (for redundancy reasons) further seals are provided in position B between the radial holes and the corresponding radial rods 13.
Nevertheless this configuration prevents the lead 10 from being cooled by the H2 (sealing is upstream of the lead 10).
In addition, because of the large diameter of the radial holes and stalks 14 housed therein, sealing in position A is difficult to achieve.
Moreover, testing operations to ascertain the quality of the seals are difficult to carry out, since the whole duct between positions A and B must be pressurized, this causing testing gas waste and practical difficulties.
Alternatively, the seals are mounted in position C between the duct and the lead 10 (between the generator shaft 5 and the slip ring shaft 5a) and also in this case further seals are provided (for redundancy reasons) in position B between the radial holes and the corresponding radial rods 13.
Nevertheless also in this case test operations are very difficult to be carried out, since position A must be closed and the duct between position A and B must be sealed (test operations between C and B are not feasible).
FIG. 2 shows a further generator type in which the generator shaft 5 is connected to the rotor 15 of the exciter 8 that encircles the stator 16.
In this case, traditionally the seal is provided in position D such that it is possible to cool the whole lead 10 with H2 and, in addition, also quality testing during manufacturing can be carried out in a very easy way.
Nevertheless, further quality tests to be carried out, for example during generator outages, are not possible, because position D is only accessible from the inside of the rotor 15.
In addition to the above drawbacks, both generator types could have security problems, because in case the seals in position A or C and in position B leak (for the generator of FIG. 1) or in case the seal D leaks (for the generator of FIG. 2), H2 would reach the exciter 8 (or its slip rings 12) before it is detected; this could cause very dangerous situations.